arch: arm: mpu: declare and define nxp_mpu_config as const

Declare and define nxp_mpu_config and nxp_mpu_regions
structs as const, as they are not modified in run-time.

Fixes #10320

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>